through it into _D_ and
forces up the piston. This up-and-down motion of the piston causes
continuous rotation of the wheel _L_. If the fire is hot, steam is
formed quickly, and the piston moves rapidly; if the fire is low,
steam is formed slowly, and the piston moves less rapidly.

The steam engine as seen on our railroad trains is very complex, and
cannot be discussed here; in principle, however, it is identical with
that just described. Figure 129 shows a steam harvester at work on a
modern farm.

[Illustration: FIG. 129.--Steam harvester at work.]

In both engine and turbine the real source of power is not the steam
but the fuel, such as coal or oil, which converts the water into
steam.

180. Gas Engines. Automobiles have been largely responsible for the
gas engine. To carry coal for fuel and water for steam would be
impracticable for most motor cars. Electricity is used in some cars,
but the batteries are heavy, expensive, and short-lived, and are not
always easily replaceable. For this reason gasoline is extensively
used, and in the average automobile the source of power is the force
generated by exploding gases.

It was discovered some years ago that if the vapor of gasoline or
naphtha was mixed with a definite quantity of air, and a light was
applied to the mixture, an explosion would result. Modern science uses
the force of such exploding gases for the accomplishment of work, such
as running of automobiles and launches.

In connection with the gasoline supply is a carburetor or sprayer,
from which the cylinder _C_ (Fig. 130) receives a fine mist of
gasoline vapor and air. This mixture is ignited by an automatic,
electric sparking device, and the explosion of the gases drives the
piston _P_ to the right. In the 4-cycle type of gas engines (Fig.
130)--the kind used in automobiles--the four strokes are as follows:
1. The mixture of gasoline and air enters the cylinder as the piston
moves to the right. 2. The valves being closed, the mixture is
compressed as the piston moves to the left. 3. The electric spark
ignites the compressed mixture and drives the piston to the right. 4.
The waste gas is expelled as the piston moves to the left. The exhaust
valve is then closed, the inlet valve opened, and another cycle of
four strokes begins.

[Illustration: FIG. 130.--The gas engine.]

The use of gasoline in launches and automobiles is familiar to many.
Not only are launches and automobiles making use of gas pow